The anti-CD20 chimeric monoclonal antibody rituximab (Rituxan)
has been successfully used in the treatment of many B-cell malignancies.
Clonotypic B cells circulating in multiple myeloma (MM) patients express CD20,
and it has been suggested that these cells may function as precursor cells.
Moreover, 20% of multiple myeloma patients express CD20 on their bone marrow
plasma cells (BMPC). We therefore undertook a phase II clinical study to
determine the activity of rituximab in MM patients. As part of this study, we
performed immunophenotyping in order to correlate CD20 expression on bone marrow
and peripheral blood mononuclear cells with response to rituximab therapy.

Nineteen MM patients (median age: 62 years; range: 36 to 88
years) who were previously treated (median prior therapies: 4; range: 1 to 6
therapies) received rituximab at 375 mg/m2 /wk for 4 weeks. Three months
following the start of treatment, patients were assessed for response; they
received a second course of therapy if they attained stable disease or had a
major response.

Six of 19 (32%) patients achieved either a partial response (n =
1) or had stable disease (n = 5), with a median time to treatment failure of 5.5
months (range: 3 to 21+ months). Interestingly, 5 of these 6 patients had
CD20-positive BMPC; CD20 status could not be determined for the bone marrow
tumor cells for one patient. Overall, rituximab therapy was well tolerated,
although one MM patient developed moderate hemoptysis, which was self-limiting,
after receiving one cycle of rituximab.

CONCLUSION: These studies suggest that certain MM patients, ie,
those with CD20-positive BMPC, may derive benefit from rituximab therapy.